magnetosphere-ionosphere coupling through plasma turbulence at high- latitude e-region electrojet y....
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Magnetosphere-Ionosphere Coupling through Plasma Turbulence at High-
Latitude E-Region Electrojet
Y. Dimant and M. Oppenheim
Tuesday, April 13, 2010
Center for Space Physics, Boston University
Dynamical Processes in Space Plasmas Israel, 10-17 April 2010
Outline
• Background and motivation• Anomalous electron heating• Nonlinear current; energy deposition• 3-D and 2-D fully kinetic modeling of E-region
instabilities• Anomalous conductivity• Conclusions; future work
Solar CoronaSolar Corona Solar WindSolar WindIonosphereIonosphereMagnetosphereMagnetosphere
Inner Boundary for Solar-Terrestrial System
Earth’s Ionosphere
What’s going on?
• Field-aligned (Birkeland) currents along equipotential magnetic field lines flow in and out.
• Mapped DC electric fields drive high-latitude electrojet (where Birkeland currents are closed).
• Strong fields also drive E-region instabilities: turbulent field coupled to density irregularities.– Turbulent fields give rise to anomalous heating.– Density irregularities create nonlinear currents.
• These processes can affect macroscopic ionospheric conductances important for Magnetosphere-Ionosphere current system.
Motivation
• How magnetospheric energy gets deposited in the lower ionosphere?
• Global magnetospheric MHD codes with normal conductances often overestimate the cross-polar cap potential (about a factor of two).
• Anomalous conductance due to E-region turbulence can account for discrepancy!
Strong electron heating
Reproduced from Foster and Erickson, 2000125 mV/m
(Reproduced from Stauning & Olesen, 1989)
Anomalous Electron Heating (AEH)
• Anomalous heating: Normal ohmic heating by E0 cannot account in full measure.
• Farley-Buneman, etc. instabilities generate E.
• Heating by major turbulent-field components E B is not sufficient.
• Small E|| || k|| || B, |E|||<<|E|, are crucial:
– Confirmed by recent 3-D PIC simulations.
Analyitical Model of AEH• Dimant & Milikh, 2003:
– Heuristic model of saturated FB turbulence (HMT),– Kinetic simulations of electron distribution function.
• Difficult to validate HMT by observations:– Radars:
• Pro: Can measure k|| (aspect angle ~ 1o),• Con: Only one given wavelength along radar LOS.
– Rockets: • Pro: Can measure full spectrum of density irregularities
and fields,• Con: Hard to measure E||; other diagnostic problems.
• Need advanced and trustworthy 3-D simulations!
PIC simulations: electron density
E0 x B direction
E0 d
irec
tion
3D simulations3D simulations• 256x256x512 Grid256x256x512 Grid
• Lower Altitude (more collisional)Lower Altitude (more collisional)
• Driving Field: ~4x Threshold field (150 Driving Field: ~4x Threshold field (150 mV/m at high latitudes)mV/m at high latitudes)
• Artificial eArtificial e-- mass: m mass: me:sime:sim = 44m = 44mee; ;
ExB direction (m)ExB direction (m)
E0
dir
ecti
on
(m
) B0
dir
ecti
on
(m
)
00
102
102
1020
0
410
Potential (x-y cross-section)
Potential (x-z cross-section)
4 Billion 4 Billion virtual PIC virtual PIC particlesparticles
2D looks the 2D looks the same!same!
Higher altitude 3D simulationHigher altitude 3D simulationIons: First Moment (RMS Of Vi)electrons: First Moment (RMS Of Ve)
3-D
Tem
ps
3-D
Tem
ps
2-D
Tem
ps
2-D
Tem
ps
100 105 110 115 120 125 130 135h,km
500
1000
1500
2000
2500
3000
3500radareffT
Anomalous heating
eT
iT
0T
[Milikh and Dimant, 2003] E = 82 mV/m
(comparison with Stauning and Olesen [1989])
Cross-polar cap potential
(Merkin et al. 2005)
Anomalous Electron Heating (AEH)
• Affects conductance indirectly:– Reduces recombination rate,– Increases density.
• All conductivities change in proportion.• Inertia due to slow recombination changes:
– Smoothes and reduces fast variations.
• Can account only for a fraction of discrepancy.• Need something else, but what?
Nonlinear current (NC)
• Direct effect of plasma turbulence:– Caused by density irregularities, n.
• Only needs developed plasma turbulence – no inertia and time delays.
• Increases Pedersen conductivity (|| E0)– Crucial for MI coupling!
• Responsible for the total energy input, including AEH.
Characteristics of E-region waves• Electrostatic waves nearly perpendicular to
• Low-frequency,
• E-region ionosphere (90-130km): dominant collisions with neutrals
- Magnetized electrons:
- Demagnetized ions:
• Driven by strong DC electric field,
• Damped by collisional diffusion (ion Landau damping for FB)
0 ||, k kB
ene
ini
0 0E B
en
0B
0E
20000 / BBEV
electronsions
Two-stream conditions
(magnetized electrons + unmagnetized ions)
Wave frame of reference
0E
_+_
+_ __
++ + +
_ _ _
+
+ + +
+
__ _ _
E E
0n
20000 BBEV
Ions
0n
0n
0n
Phn VV
Electrons
0B
00 BE
0E
E
E
-e
-eNLJ
Nonlinear Current
Mean Turbulent Energy Deposit
• Work by E0 on the total nonlinear current• Buchert et al. (2006):
– Essentially 2-D treatment,– Simplified plasma and turbulence model.
• Confirmed from first principles.• Calculated NC and partial heating sources:
– Full 3-D turbulence,– Arbitrary particle magnetization,– Quasi-linear approximation using HMT.
Anomalous energy deposition
jEjEjE jEjE
NC000 jEjEjE
Nonlinear current:
Vnqj
NC
jEjE
0
NC0 jE is total energy source for turbulence!
How 2-D field and NC can provide 3-D heating?
Density fluctuations in 3-D are larger than in 2-D!
3-D vs. 2-D, Densities
Nonlinear current (NC)
• Mainly, Pedersen current (in E0 direction).
• May exceed normal Pedersen current.
• May reduce the cross-polar cap potential.
• Along with the anomalous-heating effect, should be added to conductances used in global MHD codes for Space Weather modeling
E-region turbulence and Magnetosphere-Ionosphere Coupling
• Anomalous electron heating, via temperature-dependent recombination, increases electron density.
• Increased electron density increases E-region conductivities.
• Nonlinear current directly increases mainly Pedersen conductivity.
• Both effects increase conductance and should lower cross polar cap potentials during magnetic storms.
• Could be incorporated into global MIT models.
Conclusions
• Theory & PIC simulations: E-region turbulence affects magnetosphere-ionosphere coupling:– (1) Anomalous electron heating, via temperature-dependent
recombination, increases electron density.• Increased electron density increases E-region conductivities.
– (2) Nonlinear current directly increases electrojet Pedersen conductivity.
• Responsible for total energy input to turbulence.
– Both anomalous effects increase conductance and should lower cross-polar cap potentials during magnetic storms.
• Will be incorporated into a global MHD model.
Fully Kinetic 2-D SimulationsSimulations Parameters:• Altitude ~101km in Auroral region• Driving Field: ~1.5 Threshold field (50 mV/m at high
latitudes)• Artificial e- mass: me:sim = 44me; mi:sim=mi
• 2-D Grid: 4024 cells of 0.04m by 4024 cells of 0.04m• Perpendicular to geomagnetic field, B• 8 Billion virtual PIC particles• Timestep: dt = s (< cyclotron and plasma frequencies)
E2 (
V/m
)2
Time (s)
ExB direction (m) ExB direction (m)
E0 d
irec
tio
n (
m)
Threshold electric field
FB: Farley-Buneman instability
IT: Ion thermal instability
ET: Electron thermal instability
CI: Combined (FB + IT + ET) instability
1: Ion magnetization boundary
2: Combined instability boundary
High-latitude ionosphereEquatorial ionosphere
[Dimant & Oppenheim, 2004]
3-D vs. 2-D, Temperatures
• 3-D Simulations get hotter!3-D Simulations get hotter!
Electron Moments <Vx,y,z2> Ion Moments <Vx,y,z
2>
T$mp$ratur$s 0:
0.0 0.1 0.2 0.3tim$ (s)
400
600
800
1000
T (
K)
x0 y0 z0
FBI 128x128 psi=.3 M$=88m$ dx=0.08 E=140mV W$d May 17 15:43:21 2006
T$mp$ratur$s 1:
0.0 0.1 0.2 0.3tim$ (s)
300350
400
450
500
550
600
T (
K)
x1
y1
z1
FBI 128x128 psi=.3 M$=88m$ dx=0.08 E=140mV W$d May 17 15:43:21 2006
V$lociti$s 0:
0.0 0.1 0.2 0.3tim$ (s)
-1000
0
1000
2000
3000
V (
m/s
)
Vx0
Vy0 Vz0
V_hall0
V_p$d0
FBI 128x128 psi=.3 M$=88m$ dx=0.08 E=140mV W$d May 17 15:43:21 2006
V$lociti$s 1:
0.0 0.1 0.2 0.3tim$ (s)
0
50
100
150
200
V (
m/s
)
Vx1
Vy1
Vz1
V_hall1
V_p$d1
FBI 128x128 psi=.3 M$=88m$ dx=0.08 E=140mV W$d May 17 15:43:21 2006
3-D
Tem
p2-
D T
emp
Time (s) Time (s)
‘5-moment’ transport equations
0
Ch
2 / 32 / 3
1. Continuity equation :
0, (quasineutrality: )
2. Momentum equation (in neutral frame of reference) :
3. Thermal balance equation :
e i
n
nn n n
t
n Tdm q m
dt n
d Tn
dt n
V
VE V B V
ange of enthropy Frictional heatingCollisional cooling
22,
3
where: / , is fraction of collisional energy loss
n n n n
n n n
dV T T
dt t
m m m m
V
Fluid-model equations for long-wavelength waves: they do not include heat conductivity, Landau damping, etc., but contain all essential factors.